I’ve always coveted these little Nagra BMII audio mixers and but I somehow resisted the urge to purchase one on eBay. It is a 4 channel mono mixer with simple level and low cut controls. They come up regularly for around $200 since a fair number were sold (and none were likely disposed of, due to their great cost-when-new). One factor in my hesitance was that *no one is ever willing to confirm that they do in fact work, and/or *no one seems to be able to confirm how to interface them with standard pro audio equipment, and/or *no one seems to be able to confirm what the operational specs are – gain, impedance, i/o format, frequency response. The BMII was designed and built solely for use with the Nagra recorder, the most expensive, most finely crafted portable analog tape recorder ever built, and the Nagra has somewhat peculiar i/o and power requirements.
NEways… picked one up at the flea market this weekend for a few dollars; it did not come with a power supply but it did have a couple of the necessary Tuchel 6-pin cables, which will save me the trouble of hacking new jacks into the back of the unit.
The Tuchel connector was a popular audio-connector format for professional gear in Germany in the 60s and 70s; I have some obscure old Sennheisier mics that use these things. Anyhow, a quick search online revealed a Nagra recorder manual that confirmed the pinout data on the rear of the mixer:
A few things to note: pin two does NOT connect to earth, chassis, or anything else, in the BMII: it only ‘loops thru’ to the other pin 2. Ground connection is actually achieved only thru the Tuchel shell itself. The output of the BMII is unbalanced, 2K ohm; this is suitable for connection to consumer ‘RCA’ type inputs. The inputs of the BMII are 200 ohm, and they seem to be balanced, but since I can’t find a schematic for the unit and there are no input transformers in the unit I can’t be sure. The entire thing is built on turret boards and the wiring is good… not the best I have ever seen, but very good. One thing that makes me a little nervous is that all the electrolytics (and there are dozens of them) are they particular translucent-blue brand (anyone?) that always seem to be the faulty component in whatever piece of 60′s pro audio gear I happen to be servicing at the moment.
Alright so… I’m going to build a little power supply for this thing and fire it up, see what happens. If it does in fact work + sound great, I am thinking I can add a 2K:600 UTC output transformer to make the thing useful for modern interfacing. Since the BMII was designed for ultra-critical location recording of major film productions, I am hoping to be impressed with the sound… soon…